PROJECT SUMMARY This SBIR Phase I project will create and evaluate a novel, proof-of-concept low-tech Energy-Harvesting Knee Prosthesis, or Assist-Knee Prosthesis, capable of helping geriatric individuals with transfemoral limb loss more effectively and safely perform sit-to-stand-to-sit transitions. Transitioning from a sitting to a standing position is much more difficult for an individual with transfemoral limb loss, especially within the geriatric population, due to declining or absent knee and ankle musculature. When rising from a chair, these individuals heavily rely on their sound limb and upper limb strength to lift their body weight. Geriatric prosthesis users, who often have marginal and declining strength, could retain their independence if a prosthetic knee was capable of assisting with sit-to-stand-to-sit transitions. Unfortunately, the current designs of most commercially available prosthetic knees do not assist with the sit-to-stand-to-sit transition, a task that is fundamental for normal activities of daily living. The Assist-Knee design harvests the prosthesis user's potential energy during the stand-to-sit transition and appropriately returns that energy to assist with the sit-to-stand transition. While harvesting energy, Assist-Knee provides a resistive knee extension moment that imitates eccentric contraction of the quadriceps muscles for a controlled stand-to-sit. The return of energy provides knee extension, imitating concentric activity of the quadriceps muscles, for an assisted sit-to-stand. The technological innovation of this design does not require a large motor, battery, or complex system to provide external power for knee extension assist. Our low-cost, lightweight design is an alternative to externally powered prosthetic devices. Since external power is not being added to the system, Assist-Knee is not intended to replicate maximum moment output at the knee but to provide a useful percentage of knee extension moment to assist with initiation of the sit-to-stand transition. The specific aims of this project involve: designing and constructing Assist-Knee, performing bench testing of the device, and evaluating the proof-of-concept Assist-Knee in prosthesis users. Successful design and implementation of the proof-of-concept Assist-Knee will enable geriatric individuals with transfemoral limb loss to more easily complete sit-to-stand-to-sit transitions, increase independence by reducing the need for caretaker assistance, and help control the lowering of a user during the stand-to-sit maneuver. The commercial application of Assist-Knee is a low-cost, lightweight prosthetic alternative to assist individuals with transfemoral limb loss within the geriatric population to independently complete activities of daily living.